In electrical machining of this type, of which the most typical form utilizes a succession of electroerosive electrical discharges and is commonly called "wire-cutting EDM" or "traveling-wire EDM", the wire electrode is continuously advanced from a supply side, e.g. in the form of a wire-supply reel, to a takeup side, e.g. in the form of a wire-takeup reel, through a machining zone in which the workpiece is positioned. The machining zone is flushed with a machining liquid, typically distilled water or a liquid dielectric (in electrical discharge machining or EDM) or a liquid electrolyte (in electrochemical machining or ECM), or a weakly conductive liquid electrolyte (in electrochemical-discharge machining or ECDM). The workpiece is juxtaposed with the axially traveling wire across the machining zone between a pair of guide members which serve to stretch the traveling wire for positioning it precisely in a predetermined machining relationship with the workpiece. A series of electrical pulses are applied across a machining gap formed between the workpiece and the traveling electrode to effect time-spaced electrical discharges at this gap to electroerosively remove material from the workpiece in the typical EDM mode of wire-cutting process. In the ECDM mode of a wire-cutting process, the electrolytic material solubilization is also utilized in addition to the EDM action. The ECM mode of operation utilizes a purely electrochemical action for the workpiece material removal process and may make use of a continuous DC current.
As material removal proceeds, the workpiece is displaced relative to and transversely to the continuously traveling wire electrode, typically under numerical control, along a predetermined path to generate a desired pattern of cut in the workpiece. The continuous advancement or travel of the wire is effected typically by traction drive rollers disposed at a location between the guide member on the downstream side and the wire takeup means. A desired tension is established in the traveling wire typically by providing brake rollers at a location between the guide member on the upstream side and the wire supply means.
It is known that the achievement of a satisfactory machining accuracy requires the use of a wire electrode as thin as 0.05 to 0.5 mm in diameter. This requirement has heretofore imposed restrictions in machining performance. Such a thin wire, given a desired tension, tends to be broken when arcing or short-circuiting with the workpiece takes place occasionally. Thus, the machining efficiency or removal rate hitherto achievable has been limited to an unsatisfactory level since the wire breakage is unavoidable when an enhancement of machining efficiency or an increase in the rate of relative displacement is attempted.
In my prior and copending U.S. patent application Ser. No. 60,346 filed July 25, 1979, now U.S. Pat. No. 4,321,450, divided from Ser. No. 796,369 filed May 12, 1977, now U.S. Pat. No. 4,205,213 issued May 27, 1980, it has been pointed out that these drawbacks are effectively eliminated or alleviated by imparting a vibration to the traveling wire electrode in the region of the machining gap and in a direction transverse to the axis of the traveling wire electrode, the vibration being of a frequency not lower than 100 Hz. The frequency of the vibration is preferably in a range between 1 and 50 kHz and the amplitude is preferably in a range between 1 and 5 microns or .mu.m but may be as large as slightly smaller than the size of the machining gap, say 50 microns or .mu.m. The vibration is imparted to the wire electrode in a direction, transverse to the axis of the traveling wire electrode so that an undulating oscillatory motion with more than two nodes and antinodes or loops is provided in the wire traveling between the two guide members positioned at opposite sides with respect to the workpiece. By imparting a vibration or undulating oscillatory motion to the wire electrode stretched and axially travelling between a pair of wire guide members, it appears that a pumping action is generated in the machining zone to facilitate removal or carrying-away therefrom of machined products, i.e. chips and gases and, of even more importance, a dispersive production of successive discharges in the machining zone over the entire workpiece thickness is assured, thus not permitting the discharges to be concentrated on a single point or region of the wire electrode traveling through the workpiece. It is also conceivable, though possibly less important, that the contact resistance on the guide members and other contact portion with the wire electrode is substantially reduced. Means for imparting the vibration to the traveling wire electrode is preferably in contact with a wire guide member for the electrode and may be an electromagnetic or a sonic or ultrasonic vibrator. The vibrator means may be a magnetostrictive or piezoelectric vibrator. A control system is preferably provided, in operation of the apparatus, to respond to the machining state in the gap and to cause a parameter of the vibration to be modified in response to the state. The vibrator means are preferably arranged to be cooled by a coolant fluid. In the use of the apparatus, the machining liquid is advantageously supplied to the wire by flowing through the location where the vibrating end of the vibrator arrangement comes in contact with the wire so that the heat generated at the vibrating body is sufficiently dissipated to avoid a detrimental heating of the wire electrode. It is also preferable to keep the plane of the wire vibration coincident with the direction of the relative displacement of the workpiece to the traveling wire electrode.
In my prior U.S. patent application Ser. No. 121,662 filed Feb. 15, 1980, now U.S. Pat. No. 4,358,655 issued Nov. 9, 1982 it has also been pointed out that the vibration is imparted to the traveling wire electrode preferably at two opposite locations on one and the other sides of the workpiece, respectively, the vibrations in the two locations being each in a direction transverse to the axis of the traveling wire electrode and of a frequency not lower than 100 Hz and preferably of different frequencies.
The vibrations are imparted at two locations opposed with respect to the workpiece to the wire electrode each in a direction transverse to the axis of the traveling wire electrode so that they are superimposed upon one another to create a composite undulating oscillatory motion with more than two nodes and anitnodes or loops in the wire traveling between the two guide members positioned at opposite sides with respect to the workpiece and each outside of each location at which the vibration is applied. By imparting a vibration to the traveling wire electrode at both sides of the workpiece through whichit is passed in a traveling-wire electroerosion system, it has been found that an enhanced improvement in the removal rate is attained, this being especially noticeable when workpieces of a greater thickness, say, more than 10 mm are machined. Thus, a much intensified pumping action appears to be generated in the machining zone to facilitate removal or carrying-away therefrom of machined products, i.e. chips and gases and, of even greater importance, a highly effective dispersive production of successive discharges in the machining zone over the entire workpiece thickness is assured, thus more favorably restraining the discharges from being concentrated on a single point or region of the wire electrode traveling through the workpiece. The vibrations imparted at the two opposed locations with respect to the workpiece are preferably of different frequencies, such that a beat or the periodic variation in amplitude of a wave, that is the superposition of the corresponding two simple harmonic waves of the different frequencies is produced in the traveling wire electrode. This arrangement has been found to be far more advantageous to facilitate and enhance the removal of machining chips and other gap products while suppressing the temperature rise of the machining workpiece.
The two vibration means are preferably positioned at their respective locations so as to provide the respective vibrations in directions transverse to each other, for example, one in the direction of x-axis and the other in the direction of y-axis, the axes along which the workpiece is displaced relative to the traveling wire electrode by the aforementioned displacement means, e.g. a numerically controlled drive means. Each of the vibrator means is preferably in contact with a wire guide member for the electrode and may be an electromagnetic or a sonic or ultrasonic vibrator. Each vibrator means may be a magnetostrictive or piezoelectric vibrator. The vibrators may be connected for energization with respective resonant circuits each connected across the machining gap. A control system is preferably provided, in operation of the apparatus, to respond to the machining gap and to cause a parameter of the vibrations to be modified in response to the gap state. The vibrator means are preferably arranged to be cooled by a coolant fluid. In the use of the apparatus, the machining liquid is advantageously supplied to the wire by flowing through the location where the vibrator end of each vibrator arrangement comes in contact with the wire or by flowing in contact with the body of each vibrator so that the heat generated at the vibrating body is sufficiently dissipated to avoid a detrimental heating of the wire electrode. It is also sometimes desirable to keep the plane of the wire vibration coincident with the direction of the relative displacement of the workpiece to the traveling wire electrode.
While a substantial improvement in machining performance is obtained by imparting a vibration or vibrations to the traveling wire electrode in the manner described, it has now been observed that this technique may entail a certain disadvantage especially in conjunction with the contour-machining feed or relative displacement required between the workpiece and the traveling wire electrode. Specifically, where the contour-machining path includes locations at which the direction of transverse advance of the vibrating wire electrode must be changed at any angle, an undesirable drop in the machining accuracy may occur at those locations on the workpiece.